Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The învention pertains generally to control circuits
for the actuation of solenoid devices and is more par-
ticularly directed to a driver circuit for controlling
the current to the solenoid of an electro-magnetic fuel
injector.
For the precise control of the air/fuel ratio of an
internal combustion engine electronic fuel injection
systems are being widely used today. These systems have
an electronic control unit which generates a pulse width
signal w~ose duration is indicative of the quantity of
fuel to be input to the engine by sensing the instan-
taneous engine operating parameters. The pulse width
signal controls the opening and closing of a plurality of
electro-magnetic fuel injectors having solenoid actuated
valves.
During the energized state of the injectors pres-
surized fuel is metered through the valve by an orifice
or nozzle producing a predetermined flow rate. The flow
rate multiplied by the actual open time of the injector
will determine the quantity of fuel input to the engine.
The precision of fuel flow control will be dependent upon
matching the actual opening and closing times of the
injectors to the duration of the control pulse from the
electronic control unit.
Generally, solenoid driver circuits have been used
to interface the electronic control unit and the fuel
injectors. The solenoid driver circuits attempt to more
equally match the mechanical operating characteristics of
the valves to the electrical pulse width by regulating
the current and voltage levels necessary to Gpen and
close thP solenoids~ The solenoid driver circuits may
additionally ga~e the pulse width to individual groups of
injectors.
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One such prior art circuit that is known to be
advantageous for the operation of this type of solenoid
injector includes circuitry for generating a peak current
through the coil of the injec.or solenoid in excess of
S the amount that is needed to open the valve or "pull in"
the armature and then switching to a holding current
which is above that at which the armature will close the
valve or "drop out." This method insures the rapid
opening of the injector by the peak current and the
efficient use of power thereafter by the lower holding
current which also allows the injector to close more
quickly.
However, not all solenoid injectors will operate
consistently at their "pull in" current value and there-
fore the peak value must be set to a point sufficientlyhigh enough to ensure all injectors will be open every
time it is reached. This is because it is difficult to
predict the exact opening time of an injector by
assigning a certain current value as the value may change
with age and will not be the same for different
injectors. However, the opening of the val~e can be
predicted to occur sooner at higher battery voltage or
later at lower battery voltage because the L/R time
constant controlling the rise in current in the coil of
the solenoid to the peak value will not change. Thus, to
minimize inherent mechanical error, the valve should be
opened as quickly as possible with the maximum battery
voltage available.
In automotive systems differences in battery voltage
may occur under air conditioning loads, cold cranking,
and other conditions. The lack of regulation in the
battery voltage will cause the injectors to open and
close at undeterminable points. This will cause an error
which is uncalibrated for in the electronic control unit
and which is dependent upon the physical characteristics
of the solenoids and variations in the applied source
voltage.
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At least two previous systems have attempted to overccme
this problem. Illustrative of one solution is U.S. Patent 3,725,678
issued to Reddy which is commonly assigned with the present appli-
cation. Reddy eliminates the opening and closing time differentials
due to battery voltage change by closely regulating the voltage
applied across the coils of the solenoids during the time the current
is building to a peak and during the holding current period.
While advantageously equalizing the solenoid operational
characteristics the regu]ated voltage initially applied to the coils
is necessarily less than the full battery voltage available. As
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previously mentioned the application of the maximum battery voltage
available at the opening will permit the solenoid to operate in the
shortest period of time and minimize the inherent mechanical error.
This rapid operation is becoming increasingly important as the con-
trol pulse widths become shorter and linearity for -the injector
operation is necessitated into the millisecond operating range.
Another U.S. Patent 4,092,717 is.sued to Di Nunzio dis-
closes a system for sta~ilizing the opening time of an electro-
ma~netic ;njector against variations in vehicle battery voltage.
Di Nunzio teaches measuring a battery dependent delay from the ter-
mination of the control pulse until an empirically determlned "drop
out" current is reached and then subtracting that value from the sub-
sequent injection cycle which initiates at an empirically determined
"pull in" current.
In this type of system the pulse width indicating an
error is not compensated according to deviation presently occurring
but by that of a previous pulse width. Counting error in such a
system may tend to accumulate
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rather than cancel. More importantly empirical measure-
ment of two changing values of current for each injector
must be made.
Further to more adequately equalize the closing
times of the solenoid injectors the prior art has used a
Zener diode connected between the coils of the injectors
and a reference voltage. The Zener upon the disruption
of the current supply to the coils will allow the voltage
on the coil to build until it exceeds the breakdown
voltage o the diode. Thereafter, the Zener produces a
controlled rate of decay or dissipation of the stored
energy so that the injectors close at the same rate every
time. The time of closure after termination of the
control pulse is however a function of the holding
current level. The energy stored in the coils at turn
off is l/2Li2 and thus varies as the square of the
holding current. The greater the holding current the
longer it will take at the controlled decay rate to reach
the drop out current.
SUMMARY OF THE INVENTION
The invention provides a method and apparatus for
controlling a solenoid such that the operational times of
-the armature upon energization and release are stabilized
in order to more nearly match the mechanical actuation of
the solenoid to the pulse width of a control signal. In a
preferred implementation the apparatus controls the
current through the coil of a solenoid actuating an elec-
tromagnetic fuel injector.
The apparatus comprises a solenoid driver circuit
including a means responsive to the presence of a control
pulse signal for operably connecting a coil of a fuel
injector to a voltage source through a current path and
for controlling the conductance of the current path so
that the current through the coil reaches a peak current
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level in excess of the pull-i,n current of the in,ector.
The connecting means thereafter controls the conductance
of the current path to reduce the current through the
coil to a ho],ding current level in excess of the drop-out
level of the injector. Further included are means for
discharging the energy stored in the coil at a controlled
or constant rate upon the termination of the control signal.
In its method aspect, the invention relates to a
method of controlling the actuation of a solenoid having
a coil comprising the steps of: energizing the solenoid
in response to the presence of a control signal by
electrically coupling the coil to a voltage supply until
a peak current level through the coil is obtained, and
by altering the electrical coupling between the voltage
supply and the coil to reduce the current therethrough
to a holding current level once the peak current level is
reached; regulating the holding current level as a
function of the variations in voltage of the voltage
supply; and dissipating, at a controlled rate, the energy
stored in the coil as a result of the holding current
level after the termination of the control signal.
The driver circuit further comprises a means for
regulating the hold current at a level which will cause
the turn off time of the solenoid to be oppositely varied
by an amount equivalent to the amount the turn on time of
the solenoid is varied by changes in the battery voltage.
This regulation means will equalize the errors in the
opening and closing time of the injector for variations
in the battery voltage.
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In a preferred implementation the connecting means
comprises a driver amplifier operably connected at its
output to the common terminal of a pair of bilateral switches
which are connected at their other contacts to the control
terminals of a pair of driver transistors. The bilateral
switches are closed by a select signal which will connect
only one of the driver transistors according to the logic
level input from a select line. With this configuration
grouping of the injectors may be facilitated and circuit
costs reduced by driving more than one injector per driver.
Either of the injectors can be assigned to multiple groups
as long as the control pulses to both injectors of the
pair are not to be overlapped. Timing and operational
sequencing are easily accomplished in this manner.
Therefore it is an object of the invention to
provide a dual injector driver whereby one of the injectors
may be selected for energization with a select signal and
controllably driven with a control pulse.
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Further it is an object of the invention to provide
such a driver wi~h stabilized opening and closing times
to more nearly match the duration of the control signal
pulse to the injected Euel quantity.
Further still it is an object of the invention to
provide the stabilization by regulating the hold current
in proportLon to the changes in battery voltage.
These and other features, advantages and aspects of
the invention will be more fully understood and better
explained if a reading of the Detailed Description is
undertaken in conjunction with the appended drawings
wherein.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a detailed electrical schematic diagram
of a dual solenoid driver circuit constructed in accor-
dance with the invention;
Figure 2a is a representative waveform diagram pic-
torially illustrating the current through the coil of a
solenoid driven by the circuit illustrated in Figure l;
and
Figure 2b is a representative waveform diagram of
the actuation of the armature of the solenoid in a timed
relationship to the current waveform illustrated in
Figure 2a.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With respect now to Figure 1 there is shown a dual
solenoid driver circuit incorporating the advantages of
the invention. The driver circuit comprises a driver
amplifier A2 whose output via a resistor R12 can be
operably coupled through analog switches Sl or S2 to the
base terminals of a pair of NPN driver transistors T2 and
T4 respectively. Upon the closure of a switch the output
of amplifier A2 will control the conductance of the asso-
ciated transistor.
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Switches Sl and S2 in the preferred form are solid
state bilateral devices with individual inputtermunals deter-
mining their state of operation. Switch Sl has its inputter-
minal connected to a select si~nal via a select terminal 12
and s~itch S2 has its input terminal connected to the inversion
of the select signal via its connection to the output of in-
verter I2. Normally, the select terminal in the absence of
a select signal is at a low logic level and switch Sl is
open and switch S2 is closed. A transition to a high logic
level at the select terminal 12 i`ndicating the presence of a
select signal will reverse these states and switch Slwill
close and switch S2 will open.
The driver amplifier A2 has a control voltage applied
to the noninverting input from a control terminal 10 via an
input resistor Rl. The contro] voltage is a pulse widthwhose
duration is indicative of the quantity of fuel to be input to
the engine. The control voltage pulse is preferably output
from an electronic control unit (ECU) of a fuel injection
system which calculates the pulse width as a function of at
least one operat;ng parameter of t~le engine. An ECU of this type
is more fully s~own in applicant's U.S. Patent No.4,212,066,
issued July 8, 1980. With reference to this patent, each
coil or inductor L2, L4 can represent a group of four parallel
fuel injectors which can be selected at 180 intervals of
engine crankshaft revolution by the signal FFS.
The control pulse voltage which is a nominal logic
level of ~5V and 0~ is divided down to a predetermined fraction of
the input voltage from the control termanalby the input resistor
~1 and a va-riable resistor R2 connected between the non-
30~ inverting input and ground.
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The driver amplifier A2 operates in a closed loop
fashion as a voltage follower by having a regulation
voltage applied at its inverting input from the junction
of a pair of divider resistors R5 and R4 connected
between the battery supply voltage +B and a sense line
16. The voltage on the sense line 16 is representative
of the current drawn through the coils of the controlled
solenoids. The regulation voltage at the inverting input
of amplifier A2 varies with the voltage of the battery
voltage +B and primarily as the voltage on the sense line
16. ~ capacitor C2 ~or controlling the slew rate of the
amplifier A2 is further connected between its output and
the invertlng input~
The battery voltage +B is coupled through a
resistor R13 to a filtering and clamping circuit
comprising the parallel configuration of a capacitor C4
and a Zener diode Z4 connected between the resistor R5
and ground. The Zener diode Z4 has a nominal breakdown
voltage of 15V and clamps the battery voltage +B to 15V
if it increases beyond this point. The filter smooths
out variations in the battery voltage and has a time
constant in excess of the maximum control pulse signal
for the circuit.
~ ach driver transistor, for e~ample transistor T2,
is connected to an associated coil or inductive load L~
at its collector and by itslemitter to a sense resistor
Rll. The other terminal of the coil L4 is coupled to the
battery voltage +B and the sense resistor Rll is grounded
at its other terminal. Similarly the driver transistor
~4 is coupled at its collector to the inductor L2 and at
its emitter to the sense resistor Rllo The other
- terminal of the inductor L2 is coupled to the battery
voltage +B, Each driver transistor T2, T4 has a "pull
down" resistGr R9, R10 respectively connected between its
base and the sense line 16.
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The configuration of the driver transistors T2, T4
and coils L4 and L2 respectively form serial current
paths through the inductors, transistors, and the sense
resistor from the battery voltage +B to ground. By
controlling the conductance of each driver transistor the
amount of current through the inductive loads and as a
consequence the operation of each solenoid can be regu-
lated. The sense resistor Rll is chosen as a low value in
the order of tenths of one ohm, to provide a voltage
indication of the current drawn through the inductance of
the coil to the sense line 16 without a large power dis-
sipation.
Importantly, the connection of the serial current
path of the coil, transistor, and battery voltage allows
the coil to charge at the maximum rate when the driver
transistor is saturated. ~he maximum charging rate will
produce the fastest opening of the injector and contri-
bute to the linearity for injector pulse widths into the
millisecond range.
Coupled to each inductor L2, L4 through blocking
diodes D4 and D2 respectively is a Zener diode Z2 with
its cathode commonly connected with the cathodes of the
blocking diodes and its anode connected to the battery
voltage +B. The Zener diode Z2 will, after each of the
power transistors is turned off allow a positive voltage
to build from the collapsing field of the energized
inductive loads until it reaches the Zener breakdown
voltage. The Zener diode Z2 thereafter will dissipate
energy from the inductor at a constant rate by regulating
the impedance apparent to the collapsing electromagnetic
field. In the configuration shown a higher breakdown
voltage will increase the xate of energy dissipation. Of
course, the breakdown vo~tage must be low enough so that
the driver transistors will not break down.
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Further coupled to the sense line 16 is a comparator
A4 which compares the voltage developed on the sense
line, received at its inverting input, to a threshold
voltage provided at its noninverting input. The non-
inverting input is connected to a junction of a pair ofdivider resistors R6 and variable resistor R8 connected
between a regulated voltage +A and ground. The junction
voltage of the divider forms part of the threshold
voltage and another part is formed by the control
terminal voltage through the serial path of resistors Rl,
R3, R7, and R8.
When the sense voltage exceeds the voltage thres-
hold, the comparator A4 will switch into a conducting
state and ground the junction of resistors R3 and R7
through its output. The function of the comparator A4
when conducting is to change the voltage levels applied
to the noninverting input of amplifier A4 and the voltage
applied to the noninverting input of amplifier A2.
Operationally, the driver circuit functions to
2~ control current through coils L4 and L2 in response to
the select.and control signal levels. The select input
is supplied with a logic signal of one of two levels and
if the level is high switch Sl will be closed and if the
signal is low the inverter I2 will close the switch S2.
Depending upon which switch is closed one of the bases of
the driver transistors T2 and T4 will be operatively
coupled to the output of the amplifier A2.
In a quiescent state with no control pulse applied,
the voltage at the inverting input of amplifier A2 is
greater than the voltage at the noninverting input and
the amplifier is off. The selected driver transistor is
therefore nonconducting. Additionally, the threshold
voltage applied to the noninverting input of amplifier A2
biases it in a nonconducting state.
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When the leading edge of a control signal pulse is
applied to the control terminal 10, amplifier A2 will
turn on and generate a high level voltage signal to the
base of the selected driver transistor. The output level
S from amplifier A2 causes the transistor to conduct and
pulls current from the battery voltage +B through the
coil, the conducting transistor, and the sense resistor
Rll.
The control voltage applied at the noninverting
input oE amplifier A2 is sufficient to cause the ampli-
fier to output a signal that will saturate the driver
transistor. The amplifier A2 acts as a voltage follower
whereby the amplifier attempts to e~ualize the voltages
applied to both of its input terminals. As more current
is pulled through the coil and saturated transistor the
voltage developed in sense resistor ~1 and hence on the
sense line will rise and begin the equalization process.
The current through the selected inductor will as a
consequence rise very quickly with a LjR time constant
until the voltage at the inverting input of the amplifier
A4 exceeds the threshold voltage applied to the non-
inverting input. This is the peak current level which
can be adjusted by the variance of resistor R8 to change
the threshold voltage.
25At the time the current exceeds the peak threshold
amplifier A4 will switch into a conducting state and
resistor R3 will be paralleled with the var;able resistor
R2 through the output of the amplifier to ground.
Further, the resistor R7 will be grounded through the
output of the amplifier A4 to lower the threshold voltage
applied to the noninverting input in order to latch the
amplifier A4 into a conduc~ing state.
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The swit~hing of the resistor R3 into the circuit
will cause the voltage at the noninverting input of
driver amplifier A2 to fall. The driver amplifier A2
will start to switch off; but the action of the capacitor
C2 will slow the switching at a controlled slew rate.
Thus, the amplifier A2 will not turn the driver tran-
sistor completely off which could cause the solenoid to
drop out. The control voltage of the driver transistor
is lowered until the voltages on the inputs to the driver
amplifier A2 have equalized. This lower drive to the
transistors will produce the holding current level for
the injector. The initial holding current level can
therefore be adjusted by the variance of resistor R2.
Importantly according to the invention, as the
battery voltage -~ changes, the voltage on the inverting
input of the amplifier A2 because of resistor R5 will
also change. If the battery voltage increases the output
of the amplifier A2 and hence the holding curre~t through
the coils will decrease and conversely if the battery
voltage decreases the holding current level will
- increase.
At the trailing edge of the control pulse the
amplifier A2 will turn off and cause the selected driver
transistor to become nonconducting. The collapsing
magnetic field on the energized injector will produce an
increasing positive voltage at the collector of the
selected driver transistor. When the voltage exceeds the
Zener breakdown voltage, the diode Z2 will conduct and
dissipate the stored energy at a controlled rate.
Figures 2a and 2b illustrate the manner in which the
regulation of the holding current by variations in the
battery voltage accomplishes the stabilization of the
opening and closing times o~ a solenoid of an electro-
magnetic fuel injector. In ~igure 2a there is shown a
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waveform 1~ which represents the current through the coil
of a solenoid driven by the preferred embodiment of the
driver circuit. The waveform 18 comprises a peak current
level Ip and a holding current level IHl for an injector
turned on at the time marked pulse on and turned off at
the time marked pulse off in the figure. The pulse on and
pulse off times correspond to the leading and trailing
edges of a control pulse such as that disclosed for
transmission to terminal 10. It is seen that the current
initially after the pulse on time occurs rises exponen-
tially at the L/R ~ime constant of the coil to where the
armature or valve of the injector begins to pull in at
point 13. At this time the inductance of the coil
changes and there will be a slight dip in load current as
the actuation of the armature opens the valve. This (as
is seen in Figure 2b) is the beginning of the actual
injection or the open mode of the injector~
Thereafter the current continues to rise until the
peak current level Ip is reached to ensure the pull in
current has been exceeded. The drive circuit switches
once this level has been exceeded to limit the current to
the holding current level IHl.
When the trailing edge of the pulse occurs, the
solenoid driver circuit will cause the injector driver
transistor to turn o~f and current will be controllably
dissipated through the Zener diode ~2 at an exponential
decay rate to where the drop out current level Id is
reached. This point of time l9 corresponds to the actual
closing of the armature at edge 24 as seen in Figure 2b.
A second waveform 20 showing the coil current
present at a higher battery voltage is illustrated in
Figure 2a~ It is seen that the higher battery voltage
causes the current to ramp at a higher rate than the
waveform 18 (the L/R time constant remaining unchanged)
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and to pull in the armature sooner at point 15 such that a
time differential tl exists between the opening of the
injector with the waveform 20 at point 28 and that of the
waveform 18 at point 22. The circuit operates through
the negative feedback at the higher battery voltage to
reduce the holding current to another holding level IH2.
This lower holding current reduces the amount of energy
stored in the inductance of the coil. When the pulse
turns off, the smaller amount of energy stored allows the
Zener diode to dissipate the energy quicker. The
injector, therefore, reaches the drop out current level
Id at point 21 relatively fast. By reaching the drop out
point sooner a time difference of t2 exists between the
closing of the injector at 24 and the closing at 26.
lS If the time differentials tl and t2 are equalized by
knowing the constant rate of the discharge by the Zener
diode and the differences in the time to the peak
current, then the actual differentials in opening and
closing times can be equivalently matched by varying the
holding current as a function of battery voltage.
Preferably, the solenoid driver circuit described
controls one or more conventional fuel injectors by
generating a peak current of approximately 1.5 amps for
each injector and a hold current in excess of their 200-
300 milliamp drop out current. For such an advantageousconfiguration a 600 milliamp holding current is chosen at
a nominal battery voltage of 13.5V with a regulation of
100 milliamps/volt of deviation. The clamp voltage of
15V is chosen to eliminate the possibility of the hold
current being reduced to the point where the armature may
drop out. The filter eliminates variations in holding
current during pulse generation and large variations due
to transient conditions.
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While a preferred embodiment of the invention has
been shown, it will be obvious to those skilled in the
art that various modifications and changes may be made to
the disclosed system without departing from the spirit
and scope of the invention as defined in the hereinafter
appended claims.
What is claimed is:
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